This invention relates to an automatic focusing device for microscopes of the reflected light type, and particularly to microscopes which may be switched from bright field to dark field illumination and vice versa.
For certain uses, as for example for microscopic examination or verification of substrates or wafers in the electronics industry, there is a need for reflected-light microscopes which have automatic focusing capability, which relieves the user of the annoyance of having to refocus the focal plane on the surface of the object when scanning extensive regions of the specimen. For this purpose, automatic focusing systems have become popular, which provide an auxiliary illumination used for the automatic focusing control, this auxiliary illumination being in a special wavelength region such as the ultraviolet region, and being reflected into the ray path of the bright-field illumination of the microscope.
Such automatic focusing devices are known, for example, from Fed. Rep. of Germany Pat. No. 21 02 922 and its corresponding British Pat. No. 1,355,472, published June 5, 1974, and from Fed. Rep. Germany Offenlegungsschrift (published but unexamined application) No. 32 19 503 and its corresponding British patent application No. 2,122,045 A, published Jan. 4, 1984, and from Fed. Rep. Germany Offenlegungsschrift No. 34 46 727, published July 3, 1986.
However, until now the known automatic focusing devices have operated only with bright-field illumination. If a microscope equipped with the prior art focusing devices is switched to dark-field operation by inserting stops or changing the incident-light reflector, then the automatic focusing is no longer operative. The reason for this is that with incident-light dark-field illumination, the Kohler principle can not be maintained to the same extent as in the case of bright-field illumination, because in the case of dark-field illumination the light is conducted as an annular beam outside the objective and is focused on the object by an annular mirror placed around the object. In this case, if uniform dark-field illumination is desired for a number of objectives, the light stop is no longer focused precisely in the object plane as it is during bright-field operation. The fixed association of magnifiers and pupils with the object plane, indispensable for dependable automatic focusing, is lost.
Illuminating devices which can be switched between bright-field and dark-field illuminating modes are disclosed, for example, in Fed. Rep. German Offenlegungsschrift No. 23 31 750 and its corresponding U.S. Pat. No. 3,930,713, granted Jan. 6, 1976, and in Fed. Rep. Germany Offenlegungsschrift No. 25 42 075 and its corresponding U.S. Pat. No. 4,127,318, granted Nov. 28, 1978, and in Fed. Rep. Germany Gebrauchsmuster (Utility model) No. 79 17 232, published Sept. 20, 1979.